1. Field of the Invention
The present invention relates to a method and apparatus for signal receiving synchronization and more particularly to the method and apparatus capable of providing the signal receiving synchronization by estimating timing and frequencies for synchronization with high accuracy even when communication lines are in a bad state.
2. Description of the Related Art
In a portable radio line of a mobile radio communication system and in a satellite radio line using a TDMA (Time Division Multiple Access) system, in order for a signal receiver to successfully receive sending information transmitted from a signal sender i.e., to establish good synchronization between them, it is necessary to match signal receiving timing and frequency of a signal receiving apparatus to those of signals to be received.
Therefore, conventionally, in order for the signal receiving apparatus to provide synchronization for receiving a signal fed from the sender, a multiplexed signal for synchronization contained in a control signal sent through a radio control channel is sent from the signal sender to the signal receiver. The multiplexed signal containing frames, each being composed of two or more slots (also referred to as xe2x80x9cburstsxe2x80x9d), contains a CCS (Common Channel Signaling) signal containing a synchronization word composed of several tens of symbols inserted in one slot and a frequency estimating signal inserted in the other slot (i.e., in any of slots other than the slot into which the CCS signal is inserted). Data for controlling is inserted into a part having no synchronization word in the CCS signal.
A peak value is obtained by taking a moving average of levels of signals received through the control channel in each of the frames of the control signal, and a time when a largest peak value occurs is defined to be the time when a rough timing estimation is achieved, i.e., to be the time when a receiving signal (frequency estimating signal) is detected.
Though the frequency estimating signal is obtained by detection of the receiving signal described above, it also means that the frequency estimating signal having the frequency of the control signal affected by the state of radio is propagated. The detected frequency estimating signal is converted into spectrum data by using FFT (Fast Fourier Transform) and an estimated frequency error (i.e., a difference between the frequency assigned to the control channel by the signal receiver and the frequency of the receiving signal) is calculated and, by using the estimated frequency error, the frequency of the synchronization word in the CCS signal contained in a predetermined slot in the frame is corrected. The correlation between the received signal with its corrected frequency and a synchronization word (i.e., pre-specified bit series) is examined over a predetermined range of the signal and whether the correlated value exceeds a predetermined threshold is observed in order to check the appropriateness of the rough estimating of timing. If the correlated value exceeds the threshold, it is defined that the synchronization is established, i.e., the time when a peak correlation value occurs is defined to be the time when the timing is provided between the receiver and the sender. This timing is used for demodulation by the receiver.
The conventional method in which signals received in only one frame are used for establishing synchronization can achieve a desired purpose if radio propagation is in a comparatively good state. However, if the state of radio propagation becomes worse than expected in the conventional method, larger errors occur in the detection of received signals, estimating of the signal frequency and the correlation, thus resulting in a decrease in the accuracy of the signal synchronization process. Accordingly, it is impossible for the conventional method to satisfactorily achieve the synchronization purpose as desired.
A synchronization method for received signals that may solve such a shortcoming as described above is disclosed in European Patent Application (EP 0 809 377 A2). By referring to FIG. 12, the synchronization method disclosed in the above European Patent Application is described below.
In this method, a multiplexed signal for synchronization is sent through a radio control channel from a signal transmitter to a signal receiver. The multiplexed signal having frame structure, each being composed of two or more slots (also referred to as xe2x80x9cburstsxe2x80x9d), contains a CCS (Common Channel Signaling) signal containing a synchronization word composed of several tens of symbols inserted in one predetermined slot and a frequency estimating signal in the other predetermined slot (i.e., in any of slots other than the slot into which the CCS signal is inserted).
A peak value is obtained by taking a moving average of levels of signals received through the control channel in each of frames of the control signal, and the time when a largest peak value occurs is defined to be the time when a rough timing estimation is achieved, i.e., to be the time when a receiving signal (i.e., frequency estimating signal) is detected. By using the roughly estimated timing, the receiving signal (i.e., frequency estimating signal) is detected. This rough estimating of the timing is performed in the signal receiving synchronization method disclosed in the European Patent Application as follows.
The measurement of peak values of signal levels obtained by taking a moving average of levels of signals received is performed on signals received in two frames (at Step SD1 in FIG. 12) and two or more peak values corresponding to each other are selected and, if a time interval between these peak values is appropriate, it is presumed that the timing for synchronization is provided by using the roughly estimated timing. With the use of this roughly estimated timing, in the same manner as in the conventional method, the receiving signal is detected (i.e. xe2x80x9cDetected or notxe2x80x9d, at Step SD2).
When it is impossible to perform the rough estimating of the timing for synchronization and also impossible to detect the receiving signal (in the case of xe2x80x9cNot Detectedxe2x80x9d at Step SD2 in FIG. 12), in the same manner as in the conventional method, the same processing as above is carried out on the next frequency assigned to the control channel (i.e. xe2x80x9cTo next CCS frequencyxe2x80x9d, at Step SD3).
The frequency is estimated by the FFT-conversion of the frequency estimating signal obtained based on roughly estimated timing and by detecting the receiving signal (in the case of xe2x80x9cDetectedxe2x80x9d at Step SD2). The estimating of this frequency is made in the method disclosed in the European Patent Application as follows. That is, the above estimating of the frequency is performed on signals received in two frames of the control signal in accordance with a carrier to noise ratio (C/N ratio). If the estimating of the frequency is made twice, resulting values are averaged to finally obtain the estimated frequency (i.e. xe2x80x9cEstimating of frequency errorxe2x80x9d, at Step SD4).
The frequency estimating error at the time of receiving signals is estimated and the frequency of the synchronization word in the CCS signal contained in the next slot is corrected by using the estimated above frequency error. Then, the correlation between the synchronization word with its frequency corrected and the synchronization word (synchronization word known to the signal receiver) set to the control channel from the signal receiver side is examined over a predetermined range of the signal (i.e. xe2x80x9cJudgement of correlation between signalsxe2x80x9d, at Step SD5).
By checking whether the peak correlation value exceeds a predetermined threshold value, a fine estimating is made instead of the rough estimating of the timing. If the peak correlation value described above exceeds the threshold value (in the case of xe2x80x9cLarge correlation valuexe2x80x9d at Step SD6), it is presumed that the synchronization between the receiver and the sender has been established, i.e., the time when the peak correlation value is obtained is considered to be the time when the timing for the signal receiving is provided (i.e. xe2x80x9cTrackingxe2x80x9d, at Step SD7).
Moreover, if the peak correlation value does not exceed the predetermined threshold value (in the case of xe2x80x9cSmall correlation valuexe2x80x9d at Step SD6), in the same manner as in Step SD3, the same processing is performed on the next frequency assigned to the control channel (i.e. xe2x80x9cTo next CCS frequencyxe2x80x9d, at Step SD8).
In the signal receiving synchronization method disclosed in the above European Patent Application, the appropriateness of the time interval is checked and, if it is appropriate, the time point when the peak value occurs is considered to be the roughly estimated value of the timing. In such methods for estimating the timing as above, when part of the signal is greatly attenuated by an influence of phasing and the like, it is impossible to make the exact estimate of the timing and there is an increasing possibility that the receiving signal (frequency estimating signal) is not detected. For example, there is a case where, though an appropriate peak value is undetected in the first frame out of two frames of the control signal, a peak value is detected in the second frame with the same timing with which the peak value has been undetected in the first frame.
Also, in the signal receiving synchronization method disclosed in the above European Patent Application, the estimating of the frequency is made by averaging frequencies of signals obtained in two frames. However, when the estimating of the frequency is made by averaging frequencies of signals obtained in two frames, if the frequency distribution in time sequence differs between frequencies of signals obtained in the first frame and in the second frame, probability of error in the estimating of frequencies can be increased accordingly. For example, let it be assumed that frequency data in the case without noise is {0, 0, 0, 3, 0, 0, 0, 0}, frequency data in the case with noise is {1, 1, 2, 4, 2, 1, 1, 1} in the first frame and is {1, 1, 2, 3, 1, 1, 2, 4} in the second frame and, when the frequency is to be estimated assuming that the frequency data in time sequence is 0, 1, 2, 3, 4, 5, 6, 7, if the frequency is to be averaged, the estimated frequency in the first frame is estimated to be xe2x80x9c3xe2x80x9d, and the estimated frequency in the second frame is estimated to be xe2x80x9c3.5xe2x80x9d. Thus, in the method disclosed in the above European Patent Application, the possibility of non-detection of the receiving signal and the probability of the error in estimating of frequencies are increased, causing a disadvantage of unsuccessful signal receiving synchronization when the communication lines are in a bad state, in particular.
In view of the above, it is an object of the present invention to provide a synchronization method for received signals and apparatus capable of increasing accuracy of detection of a receiving signal, of estimating of a frequency for synchronization and of improving reliability in the synchronization.
According to a first aspect of the present invention, there is provided the signal receiving synchronization method for establishing synchronization between a signal sender and a signal receiver based on a frequency estimating signal detected based on a predetermined number of peak values, in each frame of a control signal containing the frequency estimating signal and a synchronization word, obtained by taking a moving average of a train of the control signals sent from the signal sender to the signal receiver and based on the synchronization word detected following the frequency estimating signal, including steps of:
synthesizing peak values corresponding to each other in two frames in the control signal; and
detecting the frequency estimating signal based on a largest synthesized peak value out of synthesized peak values.
In the foregoing, a preferable mode is one that wherein includes steps of:
detecting a candidate for the frequency estimating signal based on the largest synthesized peak value obtained by synthesizing peak values obtained in two frames, and detecting, based on the candidate for the frequency estimating signal, the frequency estimating signal corresponding to a candidate for the frequency estimating signal in a third frame existing after the two frames.
Also, a preferable mode is one wherein the candidate for the frequency estimating signal corresponding to the largest synthesized peak value in the third frame existing after the two frames of the control signal is detected by taking into account an amount of a shift in timing obtained by calculation of a correlation value in the third frame existing after the two frames if the correlation value between a detected synchronization word with its frequency corrected using an estimated receiving frequency error and a synchronization word set by a signal receiving synchronization apparatus and being known to the signal receiver is larger than a threshold value and by not taking into account the amount of a shift in the timing obtained by calculation of the correlation value in the third frame existing after the two frames if the correlation value is smaller than the threshold value.
According to a second aspect of the present invention, there is provided a signal receiving synchronization method for establishing synchronization between a signal sender and a signal receiver based on a frequency estimating signal detected based on a predetermined number of peak values, in each frame in a control signal containing the frequency estimating signal and a synchronization word, obtained by taking a moving average of a train of the control signals sent from the signal sender to the signal receiver and based on a synchronization word detected following the frequency estimating signal, including steps of:
detecting a largest synthesized peak value and a next largest peak value out of synthesized peak values obtained by synthesizing peak values, corresponding to each other, obtained in two frames in the control signal,
detecting the frequency estimating signal for the largest synthesized peak value detected and the next largest synthesized peak value detected, and
establishing synchronization between the signal sender and the signal receiver based on two frequency estimating signals detected and the synchronization word to be detected following two frequency estimating signals detected.
In the foregoing, a preferable mode is one that wherein includes steps of detecting, based on peak values corresponding to the largest synthesized peak value and the next largest synthesized peak value obtained in the third frame existing after the two frames in the control signal, the frequency estimating signals corresponding to each of both synthesized peak values and establishing synchronization between the sender and the receiver based on each of frequency estimating signals detected and the synchronization word to be detected following the frequency estimating signal detected in the third frame existing after the two frames in the control signal.
Also, a preferable mode is one wherein the peak value is smaller than the largest peak value and the next largest peak value obtained in a frame of the control signal and is a peak value obtained in a predetermined order from the largest peak value.
According to a third aspect of the present invention, there is provided a signal receiving synchronization apparatus for establishing synchronization between a signal sender and a signal receiver based on a frequency estimating signal detected based on a predetermined number of peak values, in each frame in a control signal containing the frequency estimating signal and a synchronization word, obtained by taking a moving average of a train of the control signals sent from the signal sender to the signal receiver and based on the synchronization word detected following the frequency estimating signal, including:
synthesizing means for synthesizing peak values, corresponding to each other, obtained in two frames of the control signal; and
detecting means for detecting the frequency estimating signal based on a largest synthesized peak value out of synthesized peak values synthesized by the synthesizing means.
In the foregoing, a preferable mode is one that wherein includes first detecting means for detecting a candidate for a frequency estimating signal based on the largest synthesized peak value obtained in the two frames and second detecting means for detecting the frequency estimating signal corresponding to a candidate for the frequency estimating signal obtained in a third frame existing after the two frames based on a candidate for the frequency estimating signal detected by the first detecting means.
Also, a preferable mode is one wherein the first detecting means includes estimating means for a receiving frequency error based on the largest synthesized peak value detected in the two frames and candidate detecting means for detecting a candidate for the frequency estimating signal, if the correlation value between the detected synchronization word with its frequency corrected using the estimated receiving frequency error and the synchronization word set by the signal receiving synchronization apparatus and being known to the signal receiver is larger than a threshold value, by taking into account an amount of a shift in timing obtained by calculation of the correlation value in the third frame existing after the two frames and, if the correlation value is smaller than the threshold value, by not taking into account the amount of a shift in the timing obtained by calculation of the correlation in the third frame existing after the two frames.
Also, a preferable mode is one wherein the synthesizing means has a memory to store the peak value which is smaller than the largest peak value and the next largest peak value obtained in a frame in the control signal and which is a peak value obtained in the predetermined order from the largest peak value and wherein the synthesizing means is used to synthesize peak values stored in the memory and corresponding peak values obtained in a subsequent frame in the control signal.
According to a fourth aspect of the present invention, there is provided a signal receiving synchronization apparatus for establishing synchronization between a signal sender and a signal receiver based on a frequency estimating signal detected based on a predetermined number of peak values, in each frame in a control signal containing the frequency estimating signal and a synchronization word, obtained by taking a moving average of a train of the control signals sent from the signal sender to the signal receiver and based on the synchronization word detected following the frequency estimating signal, including:
first detecting means for detecting a largest synthesized peak value and a next largest synthesized peak value out of peak values obtained by synthesizing peak values, corresponding to each other, in two frames in said control signal;
second detecting means for detecting the frequency estimating signals corresponding to each of both the synthesized peak values based on the largest synthesized peak value and the next largest synthesized peak value detected by the first detecting means; and
synchronizing means for establishing synchronization based on two frequency estimating signals detected by the second detecting means and a synchronization word detected following the two frequency estimating signals.
In the foregoing, a preferable mode is one that wherein includes third detecting means for detecting the frequency estimating signals corresponding to each of both synthesized peak values detected in the third frame existing after the two frames based on the largest synthesized peak value and the next largest synthesized peak value obtained in the third frame existing after the two frames and synchronizing means for establishing synchronization based on each of frequency estimating signals detected by the third detecting means and synchronization words detected in the third frame following the frequency estimating signal.
Also, a preferable mode is one wherein the first detecting means is provided with a memory to store the peak value which is smaller than the largest peak value and the next largest peak value obtained in a frame in the control signal and which is a peak value obtained in a predetermined order from the largest peak value and wherein the synthesizing means is used to synthesize peak values stored in the memory and corresponding peak values in a frame subsequent to the frame in the control signal.